On the effect of prestin on the electrical breakdown of cell membranes

Lipid-mediated prestin organization in outer hair cell membranes and its implications in sound amplification

Prestin is a high-density motor protein in the outer hair cells (OHCs), whose conformational response to acoustic signals alters the shape of the cell, thereby playing a major role in sound amplification by the cochlea. Despite recent structures, prestin's intimate interactions with the membrane, which are central to its function remained unresolved. Here, employing a large set (collectively, more than 0.5 ms) of coarse-grained molecular dynamics simulations, we demonstrate the impact of prestin's lipid-protein interactions on its organization at densities relevant to the OHCs and its effectiveness in reshaping OHCs. Prestin causes anisotropic membrane deformation, which mediates a preferential membrane organization of prestin where deformation patterns by neighboring copies are aligned constructively. The resulting reduced membrane rigidity is hypothesized to maximize the impact of prestin on OHC reshaping. These results demonstrate a clear case of protein-protein cooperative communication in membrane, purely mediated by interactions with lipids.

Complex nonlinear capacitance in outer hair cell macro-patches: effects of membrane tension

Outer hair cell (OHC) nonlinear capacitance (NLC) represents voltage sensor charge movements of prestin (SLC26a5), the protein responsible for OHC electromotility. Previous measures of NLC frequency response have employed methods which did not assess the influence of dielectric loss (sensor charge movements out of phase with voltage) that may occur, and such loss conceivably may influence prestin's frequency dependent activity. Here we evaluate prestin's complex capacitance out to 30 kHz and find that prestin's frequency response determined using this approach coincides with all previous estimates. We also show that membrane tension has no effect on prestin's frequency response, despite substantial shifts in its voltage operating range, indicating that prestin transition rate alterations do not account for the shifts. The magnitude roll-off of prestin activity across frequency surpasses the reductions of NLC caused by salicylate treatments that are known to abolish cochlear amplification. Such roll-off likely limits the effectiveness of prestin in contributing to cochlear amplification at the very high acoustic frequencies processed by some mammals.

Intra-arterial Injection of Lidocaine as a Cell Sensitizer during Irreversible Electroporation

PURPOSE

To investigate whether intra-arterial injection of lidocaine enhances irreversible electroporation (IRE) in a liver model.

MATERIALS AND METHODS

Conventional IRE (C-IRE) and lidocaine-enhanced IRE (L-IRE) were performed in 8 pig livers. Protocol 1 (tip exposure and electrode distance of 2.0 cm each) and protocol 2 (increased tip exposure and electrode distance 2.5 cm each) were used. Animals were sacrificed 3 hours after IRE. Study goals included electrical tissue properties (eg, current, conductivity) during IRE, geometry of IRE zones analyzed using computed tomography and magnetic resonance imaging (eg, volume and sphericity index), degree of acute liver damage, and irreversible cell death analyzed using microscopy (hematoxylin and eosin staining and terminal deoxynucleotidyl transferase deoxyuridine 5-triphosphate nick end labeling). Statistical comparisons were performed using the paired t test and Wilcoxon test.

RESULTS

All treatments were performed without adverse events. Electrical tissue properties were not significantly different between C-IRE and L-IRE. For protocol 1, the diameter of the largest sphere within the IRE zone was significantly larger for L-IRE than for C-IRE (25.0 ± 4.7 mm vs 18.4 ± 3.1 mm [P = .013]). For protocol 2, the volume of IRE zone was significantly larger for L-IRE compared with C-IRE (46.0 ± 5.4 cm³ vs 22.6 ± 6.4 cm³ [P = .018]), as well as the diameter of the largest sphere within the IRE zone (27.1 ± 2.2 mm vs 19.8 ± 2.3 mm [P = .020]). For protocol 1, a significantly higher degree of irreversible cell death was noted for L-IRE than for C-IRE (1.8 ± 1.0 vs 0.8 ± 1.0 [P = .046]).

CONCLUSIONS

Intra-arterial injection of lidocaine can enhance IRE in terms of larger IRE zones and an increase of irreversible cell death.

Simultaneous zygotic inactivation of multiple genes in mouse through CRISPR/Cas9-mediated base editing

In vivo genetic mutation has become a powerful tool for dissecting gene function; however, multi-gene interaction and the compensatory mechanisms involved can make findings from single mutations, at best difficult to interpret, and, at worst, misleading. Hence, it is necessary to establish an efficient way to disrupt multiple genes simultaneously. CRISPR/Cas9-mediated base editing disrupts gene function by converting a protein-coding sequence into a stop codon; this is referred to as CRISPR-stop. Its application in generating zygotic mutations has not been well explored yet. Here, we first performed a proof-of-principle test by disrupting Atoh1, a gene crucial for auditory hair cell generation. Next, we individually mutated vGlut3 (Slc17a8), otoferlin (Otof) and prestin (Slc26a5), three genes needed for normal hearing function. Finally, we successfully disrupted vGlut3, Otof and prestin simultaneously. Our results show that CRISPR-stop can efficiently generate single or triple homozygous F0 mouse mutants, bypassing laborious mouse breeding. We believe that CRISPR-stop is a powerful method that will pave the way for high-throughput screening of mouse developmental and functional genes, matching the efficiency of methods available for model organisms such as Drosophila.

Biophysical mechanisms underlying outer hair cell loss associated with a shortened tectorial membrane

The tectorial membrane (TM) connects to the stereociliary bundles of outer hair cells (OHCs). Humans with an autosomal dominant C1509G mutation in alpha-tectorin, a protein constituent of the TM, are born with a partial hearing loss that worsens over time. The Tecta(C1509/+) transgenic mouse with the same point mutation has partial hearing loss secondary to a shortened TM that only contacts the first row of OHCs. As well, Tecta(C1509G/+) mice have increased expression of the OHC electromotility protein, prestin. We sought to determine whether these changes impact OHC survival. Distortion product otoacoustic emission thresholds in a quiet environment did not change to 6 months of age. However, noise exposure produced acute threshold shifts that fully recovered in Tecta (+/+) mice but only partially recovered in Tecta(C1509G/+) mice. While Tecta(+/+) mice lost OHCs primarily at the base and within all three rows, Tecta(C1509G/+) mice lost most of their OHCs in a more apical region of the cochlea and nearly completely within the first row. In order to estimate the impact of a shorter TM on the forces faced by the stereocilia within the first OHC row, both the wild type and the heterozygous conditions were simulated in a computational model. These analyses predicted that the shear force on the stereocilia is ~50% higher in the heterozygous condition. We then measured electrically induced movements of the reticular lamina in situ and found that while they decreased to the noise floor in prestin null mice, they were increased by 4.58 dB in Tecta(C1509G/+) mice compared to Tecta(+/+) mice. The increased movements were associated with a fourfold increase in OHC death as measured by vital dye staining. Together, these findings indicate that uncoupling the TM from some OHCs leads to partial hearing loss and places the remaining coupled OHCs at higher risk. Both the mechanics of the malformed TM and the increased prestin-related movements of the organ of Corti contribute to this higher risk profile.

Functional prestin transduction of immature outer hair cells from normal and prestin-null mice

Prestin is a membrane protein in the outer hair cell (OHC) that has been shown to be essential for electromotility. OHCs from prestin-null mice do not express prestin, do not have a nonlinear capacitance (the electrical signature of electromotility), and are smaller in size than wild-type OHCs. We sought to determine whether prestin-null OHCs can be transduced to incorporate functional prestin protein in a normal fashion. A recombinant helper-dependent adenovirus expressing prestin and green fluorescent protein (HDAd-prestin-GFP) was created and tested in human embryonic kidney cells (HEK cells). Transduced HEK cells demonstrated membrane expression of prestin and nonlinear capacitance. HDAd-prestin-GFP was then applied to cochlear sensory epithelium explants harvested from wild-type and prestin-null mice at postnatal days 2-3, the age at which native prestin is just beginning to become functional in wild-type mice. At postnatal days 4-5, we investigated transduced OHCs for (1) their prestin expression pattern as revealed by immunofluorescence; (2) their cell surface area as measured by linear capacitance; and (3) their prestin function as indicated by nonlinear capacitance. HDAd-prestin-GFP efficiently transduced OHCs of both genotypes and prestin protein localized to the plasma membrane. Whole-cell voltage clamp studies revealed a nonlinear capacitance in transduced wild-type and prestin-null OHCs, but not in non-transduced cells of either genotype. Prestin transduction did not increase the linear capacitance (cell surface area) for either genotype. In peak nonlinear capacitance, voltage at peak nonlinear capacitance, charge density of the nonlinear capacitance, and shape of the voltage-capacitance curves, the transduced cells of the two genotypes resembled each other and previously reported data from adult wild-type mouse OHCs. Thus, prestin introduced into prestin-deficient OHCs segregates normally to the cell membrane and generates a normal nonlinear capacitance, indicative of normal prestin function.

Cochlear outer hair cell motility

Normal hearing depends on sound amplification within the mammalian cochlea. The amplification, without which the auditory system is effectively deaf, can be traced to the correct functioning of a group of motile sensory hair cells, the outer hair cells of the cochlea. Acting like motor cells, outer hair cells produce forces that are driven by graded changes in membrane potential. The forces depend on the presence of a motor protein in the lateral membrane of the cells. This protein, known as prestin, is a member of a transporter superfamily SLC26. The functional and structural properties of prestin are described in this review. Whether outer hair cell motility might account for sound amplification at all frequencies is also a critical question and is reviewed here.

A combined patch-clamp and electrorotation study of the voltage- and frequency-dependent membrane capacitance caused by structurally dissimilar lipophilic anions

Interactions of structurally dissimilar anionic compounds with the plasma membrane of HEK293 cells were analyzed by patch clamp and electrorotation. The combined approach provides complementary information on the lipophilicity, preferential affinity of the anions to the inner/outer membrane leaflet, adsorption depth and transmembrane mobility. The anionic species studied here included the well-known lipophilic anions dipicrylamine (DPA⁻), tetraphenylborate (TPB⁻) and [W₂(CO)₁₀(S₂CH)]⁻, the putative lipophilic anion \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{B}}{\left( {{\text{CF}}_{3} } \right)}^{ - }_{4} $$\end{document} and three new heterocyclic W(CO)₅ derivatives. All tested anions partitioned strongly into the cell membrane, as indicated by the capacitance increase in patch-clamped cells. The capacitance increment exhibited a bell-shaped dependence on membrane voltage. The midpoint potentials of the maximum capacitance increment were negative, indicating the exclusion of lipophilic anions from the outer membrane leaflet. The adsorption depth of the large organic anions DPA⁻, TPB⁻ and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{B}}{\left( {{\text{CF}}_{3} } \right)}^{ - }_{4} $$\end{document} increased and that of W(CO)₅ derivatives decreased with increasing concentration of mobile charges. In agreement with the patch-clamp data, electrorotation of cells treated with DPA⁻ and W(CO)₅ derivatives revealed a large dispersion of membrane capacitance in the kilohertz to megahertz range due to the translocation of mobile charges. In contrast, in the presence of TPB⁻ and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{B}}{\left( {{\text{CF}}_{3} } \right)}^{ - }_{4} $$\end{document} no mobile charges could be detected by electrorotation, despite their strong membrane adsorption. Our data suggest that the presence of oxygen atoms in the outer molecular shell is an important factor for the fast translocation ability of lipophilic anions.

High-throughput and real-time study of single cell electroporation using microfluidics: effects of medium osmolarity

Electroporation has been widely accepted as an important tool for the delivery of exogenous molecules into cells. Previous mechanistic studies have been carried out by observing either the average behavior from a large population of cells or the response from a small number of single cells. In this study, we demonstrated a novel microfluidic method with high throughput (up to 30 Hz) for real-time studies of single cell electroporation events. Electroporation occurred when cells flowed through a section of a microfluidic channel defined by special geometry. A CCD camera was used to monitor the response of cells starting from the onset of the electroporation. We studied the swelling of Chinese hamster ovary cells and the rupture of cell membrane during electroporation using this technique. We applied buffers with different osmolarities to investigate the effects of medium osmolarity, based on results from a population of single cells. We were able to establish the distributions of the rates of swelling and membrane rupture in the cell population. We also explored establishing the correlation between the property (the cell diameter) and the behavior (the swelling rate) of single cells. Our results indicated that the processes of swelling and rupture occurred more rapidly in the hypotonic or hypertonic buffers than in the isotonic buffer. Statistical analysis did not reveal strong linear correlation between the cell size and the swelling rate. These proof-of-concept studies reveal the potential of applying microfluidics to study electroporation of a cell population at single cell level in real time with high throughput. The limitations associated with this approach were also addressed.